Prostaglandin I.sub.2 (PGI.sub.2, prostacyclin) was found by J. R. Vane et al. in 1976. Prostaglendin I.sub.2 is synthesized from arachidonic acid via endoperoxide (PGH.sub.2 or PGG.sub.2) on vascular wall and is drawing attention as a compound having strong platelet aggregation inhibiting action, gastric acid secretion inhibiting action and vasodilation action of peripheral vessels [see C and EN, Dec. 20, 1976, P17 and S. Moncada, R. Gryglewskl, S. Bunting, J. R. Vane, Nature, 263, 633 (1976)]. ##STR1##
PGI.sub.2 has unstable exoenol structure, so that it is very unstable even in neutral aqueous solution and readly converted to 6-oxo PGF.sub.1.alpha. which has little physiological activity. It is a big drawback that PGI.sub.2 has the unstability as descrived above when used as a medicine. Further, PGI.sub.2 has drawbacks in that it is unstable in vivo and its physiological activity does not last.
A variety of derivatives which have been studied to improve the chamical stability and to promote the durability in vivo are not satisfactory.
Especially, as to the durablity of the drug, it was found to be a big drawback that .beta.-oxidation which is a metabolic pathway of fatty acid is easy to occur.
The present inventors intebsively studied based on this finding to find a very stable structure which inhibits the metabolism by .beta.-oxidation to accomplish this invention.
That is, the object of the present invention is to provide a novel PIG.sub.2 derivative having excellent stability and sustained activity in vivo.